Linear relationship between the logarithm of the equilibrium constants and the logarithm of the liquid chromatographic tographic separation factors for tautomers obtained in different solvents

Abstract

The linear relationship between the logarithm of the separation factor, a, in liquid-solid chromatogrphy (silica gel) using a less-polar solvent, such as a n-hexane- ehtyl acetate, and the logarithm of the equilibrium constant, K, for a pair of tautomers in a different solvent such as ethanol was confirmed experimentally using seventeen pairs of tautomers of steroid ketoximes and their corresponding O-methylketoximes. Theoretical considerations using a static model of the adsorption-desorption equilibrium in chromatogrpahy afforded the equation. log a  (ΔΔ G°/Δ G°) log K + (ΔΔ G°/Δ G°) log m where m is a constant relating the equilibrium constants of a pair of tautomers in two different solvent systems, namely, the solvent used in the chromatographic separation and that used in the equilibrium constant determination. The correlation coefficient obtained with 40 mol/mol % ethyl acetate in n-hexane as the chromatographic solvent system was 0.9804. The two constants of the above equation were determined as ΔΔ G°/Δ G°  0.3898 and log m  0.3999 with the same solvent system.